The present invention relates to a ceramic sheathed-type glow plug for diesel engines.
Sheathed-type glow plugs having an external ceramic heater are described, for example, in German Patent Application No. 40 28 859. In addition, metallic sheathed-type glow plugs, in which the coiled metallic filament is welded to a thermocouple, are described in, e.g. German Patent Application No. 29 37 884. In this case, the temperature in a specific cylinder may be measured during the operation of the sheathed-type glow plug by recording the thermoelectric voltage. However, a coiled metallic filament is not present in a sheathed-type glow plug having a ceramic heating element.
In addition, a sheathed-type glow plug, which has a connector element that is electrically connected to the heating element by a contacting element, is described in German Patent Application No. 198 44 347. As is evident from FIG. 1, this contacting element is designed as a spring.
In accordance with an exemplary embodiment of the present invention, the ceramic sheathed-type glow plug may have the advantage that the temperature of the heating element is measurable. In a ceramic sheathed-type glow plug, it may be possible, for the first time, to directly measure the temperature of the heating element in a selected region on the outside of the heating element, without any additional equipment. The temperature is measured in a selected region, which is small in comparison with the volume of the entire heating element. This means that the error occurring on account of a temperature distribution over a large volume may be reduced during the temperature determination. In addition, it may be advantageous that, in the sheathed-type glow plug of the present invention, the heating power may be concentrated in a selected region of the heating element without changing the cross-section of the conductive layer, so that the surface region where the heating power may be concentrated remains constant, and therefore, the surface of interaction may also be held constant. It may be further advantageous that such a ceramic, temperature-measuring, sheathed-type glow plug may be manufactured inexpensively.
Exemplary embodiments of the present invention may allow advantageous further refinements and improvements of the ceramic, sheathed-type glow plug. In particular, a suitable selection of the ceramic materials used for the different regions of the sheathed-type glow plug ensures that the mechanical stability of the heater is not significantly affected. The processing of the measured temperature values by a control unit allows the temperature to be controlled in the selected region of the heating element. Furthermore, it may be advantageous to use the sheathed-type glow plug of the present invention as a temperature sensor in passive operation, after it has fulfilled the heating function. Thus, it may be determined if the combustion in the specific cylinder is proceeding properly. It may be advantageous to control parameters relevant for the combustion, based on this information.
In comparison with the related art, the ceramic, sheathed-type glow plug according to one exemplary embodiment of the present invention may have the advantage that, because of the larger cross-section of the line, larger currents may be carried without thermally destroying the material of the contacting element. The high surface area of the contacting material may also be advantageous, since it may allow a good thermal conductivity. The elastic spring portion ensures that thermal displacements of the surrounding component parts, which are caused by different coefficients of thermal expansion, may be compensated for.
Further refinements and improvements of the ceramic, sheathed-type glow plug may be possible. For example, it may be advantageous for the contacting element to be made of graphite or a conductive ceramic powder, since these materials are corrosion-resistant. In addition, it may advantageous for just a predominant portion of the material to be graphite or a conductive ceramic or metal powder, since it may be possible to reduce the amount of expensive materials and obtain roughly the same properties. Furthermore, it may be advantageous to manufacture the sheathed-type glow plug having a contacting element according to the present invention in the manner described below, since the result is a configuration of the components in the plug housing which may prevent short circuits. Furthermore, the components may be compressed in such a manner that, on one hand, the components do not loosen and, on the other hand, the components do not burst due to the counteracting force of elastic elements (e.g. of the contacting element) being too large.